High-speed fly-by bus structures require, especially in the field of memory systems, a signal termination at the end of the bus to avoid reflections. This termination increases both, the AC (AC=alternating current) and the DC (DC=direct current), power dissipation at both ends of the bus structure, at the transmitter as well as at the terminator.
To reduce the DC power dissipation an RC termination, also referred to as a so-called AC coupled termination, can be used. This approach is, for example, used in high speed networking applications, in which a capacitor (C) is used in series with a termination resistor (R). As a consequence, the AC portion of the signal transmitted over the corresponding signal line is still terminated more or less with an impedance value of the termination resistor. However, the DC portion of the signal basically experiences an infinite impedance so that in a good approximation, no DC power is dissipated.
The series capacitor introduces additional disturbances to the signal line, for instance in the form of additional inter-symbol-interference (ISI), which is usually addressed by equalization methods on the transmitter side of the signal line of the bus in the form of an equalized driver. To be more precise, usually equalizers are added to the transmitters to compensate the additional ISI on the channel introduced by the RC termination. However, depending on the implementation, the circuits needed for equalization at the transmitter consume a considerable amount of die size.
Moreover, during long periods of inactivity, the bus may drift away making it impossible to instantaneously use the bus when needed. This is mainly caused by the capacitor introduced to the signal line of the bus in the framework of the RC termination, as a finite voltage can be generated across the capacitor caused by leakage currents and finite voltages applied to the signal lines, leading to a finite charge and hence a finite voltage at the capacitor.